Me too I would have a problem.
To avoid the problem it may help considering that:
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It is impossible by biological means to establish which creature is the first Homo sapiens.
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It is possible today to establish definitely on the basis of observable features which animal is a human and which is a chimp. The reason for this possibility is the disappearance of intermediate varieties between humans and chimps. (see Essay).
Taking account of these two facts it seems reasonable to assume that God conferred the sense of accountability to one or several human individuals at the dawn of Civilisation. Before this moment God could not ‘hold them responsible’ for the behavior he instilled in them.
At the same time He did confer “an ‘Original grace’ that freed them from ‘fleshly tendencies to sin’.” In any case this ‘Original grace’ derives from “the Incarnation of Jesus and his message of love” since, as discussed in previous postings, the history without sin is in principle possible, and also in this history God’s Son becomes flesh as completion of Creation: God’s love is one:
In the view I am proposing the first sin (the “original sin”) is one of pure pride, and leads to the loss of “Original grace”. It is the lack of Original Grace what becomes transmitted to future generations: The Christian Teaching about "transmission of the stage of need of Redemption (‘stage of original sin’), does not require any biological common descent (neither genetic nor genealogical).